South Korea said on Friday it was on heightened alert ahead of another important anniversary in North Korea, with a large concentration of military hardware amassed on both sides of the border amid concerns about a new nuclear test by Pyongyang.

US officials said there was a higher-than-usual level of activity by Chinese bombers, signaling a possible heightened state of readiness by Beijing, reclusive North Korea's sole major ally, although the officials played down concern and left open a range of possible reasons, Reuters reported.

In Russia, a Kremlin spokesman declined to comment on media reports that Russia was moving military hardware and troops toward the border with North Korea, the RIA news agency quoted him as saying.

North Korea has so far conducted five confirmed nuclear tests and numerous missile test-launches. While it has not fired missiles against other countries, it has vowed strong action – including missile attacks against South Korea – if it is invaded by the South and the US.

The North faces international pressure to abandon its arms development and nuclear program. Yet, it says the nuclear and missile programs are meant to protect the country from US “hostility,” and has escalated the arms activities, including by announcing a declared priority to develop a hydrogen bomb.

US and South Korean officials have been saying for weeks that the North could soon stage another nuclear test, something both the United States and China have warned against.

North Korea marks the 85th anniversary of the foundation of its Korean People's Army on Tuesday, an important anniversary that comes at the end of major winter military drills, South Korea's Unification Ministry spokesman Lee Duk-haeng said.

Top envoys from the United States, South Korea and Japan on North Korea are due to meet on Tuesday, South Korea's Foreign Ministry said, to “discuss plans to rein in North Korea’s additional high-strength provocations, to maximize pressure on the North, and to ensure China’s constructive role in resolving the North Korea nuclear issue”.

South Korea and the United States have also been conducting annual joint military exercises, which the North routinely criticizes as a prelude to invasion.

“It is a situation where a lot of exercise equipment is amassed in North Korea and also a lot of strategic assets are situated on the Korean peninsula because of the South Korea-US military drills,” Lee told a briefing.

“We are closely watching the situation and will not be letting our guards down,” Lee said.

US President Donald Trump on Thursday praised Chinese efforts to rein in “the menace of North Korea,” after North Korean state media warned the United States of a “super-mighty preemptive strike”.

US Defense Secretary Jim Mattis said on Friday North Korea's rhetoric was provocative but he had learned not to trust it.

On Thursday the official newspaper of North Korea's ruling Workers' Party, Rodong Sinmun, threatened the United States with a "super-mighty preemptive strike" that it said would wipe out “not only the US imperialists’ invasion forces in South Korea and its surrounding areas but the US mainland and reduce them to ashes”.